1. Field of the Invention
The present invention relates to an optical beam modulating device which is necessary in cases where, for example, halftone plate duplicate images are recorded on a recording material by controlling a light-exposure means in accordance with image signals obtained by the photoelectric of an original image, and especially in cases where halftone plate images are recorded by independently modulating a multiple number of Gauss beams (hereinafter, "Gauss beam" means a beam with an energy level which can cause exposure, the peripheral portion of the beam circle where the energy level is too low to be effective in exposure being excluded) on the basis of image signals.
2. Prior Art
The recording of halftone plate images by the relative scanning of a multiple number of light beams (lined up in a row) across the surface of a recording material, with the light beams being independently modulated on the basis of image signals, has conventionally been performed in the art. In most cases, the multiple number of light beams are obtained by installing a multiple number of totally reflective mirrors and semi-reflective mirrors, and splitting a single light beam generated by an argon laser by reflecting the light beam from the mirrors, or by splitting a single light beam using an optical beam splitter of the type described in Japanese patent application Laid-Open (Kokai) No. 52-122135. The respective light beams thus obtained are independently modulated by means of a multi-channel ultrasonic modulator and then reduced in diameter by means of a crystal optical system and directed onto the surface of a recording material.
FIGS. 3(a), 3(b) and 3(c) illustrate a conventional multi-channel ultrasonic modulator. In the Figures, numeral 1 indicates a modulator crystal, 2 indicate a crystal holder, and 3a through 3h indicate acoustic electrodes. These electrodes are positioned on a side surface of the modulator crystal 1 which runs perpendicular to Gauss beams 4a through 4h, and are installed in two rows with a one-half pitch phase difference between the two rows. The pitch p of adjacent Gauss beams in a case where the beam diameters of four gauss beams are caused to overlap in a square arrangement so that the respective beam circles pass through a central point X as shown in FIG. 4 is equal to the square root of the square of the beam radius r.sup.2 .times.2 from the Pythagorean theorem. This is considered to be approximately 0.707 times the beam diameter D of the Gauss beams. If the pitch p is smaller than this value, the point X will not be exposed to the light of the light beams. However, if the pitch of the Gauss beams 4 a through 4h is set at p, then the pitch of the acoustic electrodes 3a through 3h is also accurately set at p. The acoustic electrodes 3a through 3h generate ultrasonic waves in accordance with desired image signals for the purpose of forming images of various sizes as aggregations of dots on the recording surface. These electrodes independently modulate the Gauss beams 4a through 4h which are directed so that they traverse the respective acoustic electrodes.
FIG. 5 shows one example of the conditions of an aggregation of dots recorded on the recording surface by exposure. The recording material moves continuously in the direction indicated by arrow Y. The eight dots in row a are exposed first. Next, when the recording material has moved by pitch p in the direction indicated by arrow Y, the eight dots in row b are exposed. Next, when the recording material has again moved by pitch p in the direction indicated by arrow Y, the eight dots in row c are exposed, completing the aggregation of dots illustrated in FIG. 5. The generation of ultrasonic waves by the acoustic electrodes 3a through 3h is interrupted when rows a, b and c are exposed. When the generation of ultrasonic waves is interrupted, the direct passage of the Gauss beams 4a through 4h through the modulator crystal 1 is allowed, so that exposure takes place. On the other hand, when ultrasonic waves are generated, the direct passage of the Gauss beams 4a through 4h is blocked. In this way, the light beams are ultrasonically modulated.
However, although multi-channel ultrasonic modulators of the type mentioned above are designed so that the respective light beams can be independently modulated, the Gauss beams 4a through 4h which pass through the ultrasonic modulator 1 overlap each other. Accordingly, when a light beam corresponding to a given acoustic electrode among the acoustic electrodes 3a through 3h is modulated by the acoustic electrode, the generation of crosstalk interference which cuts the passage of the beam in the adjacent light beams on both sides is essentially unavoidable.
Furthermore, if there is not complete equality and absence of any phase shift between the pitch p of the acoustic electrodes 3a through 3h and the pitch p of the Gauss beams, or if the size of the acoustic electrodes 3a through 3h does not match the beam diameter of the Gauss beams, the abovementioned crosstalk occurs even more conspicuously. Accordingly, the formation of the acoustic electrodes 3a through 3h and the structure and arrangement of the beam splitter which determine the pitch of the Gauss beams must be maintained at a high degree of precision.
Conventionally, furthermore, the number of light beams has been limited to approximately 20 due to heat loading of the modulator crystal. For this reason, a long exposure time has been required.